The invention relates to the preparation of solutions of oligomeric methylaluminoxanes which, if desired, may contain higher alkyl groups and which contain trimethylaluminum in a free and/or complexed form; the solvents used here are aliphatic, cycloaliphatic or aromatic hydrocarbons.
Longer-chain oligomeric and/or polymeric alkylaluminoxanes of the simplified structures ##STR1## are known compounds which arc used as catalyst components in the preparation of highly active polyolefin catalysts, oligomeric methylaluminoxanes (MAO) with R=CH.sub.3 being sometimes mentioned as being preferred (DE-A-3,007,725, EP-B-O,069,951, DE-A-3,240,382, EP-A-O,170,059, DE-A-3,443,087, EP-B-O,128,046, U.S. Pat. No. 4,665,046, EP-A-O,232,595, U.S. Pat. Nos. 4,668,838, 4,665,047, EP-A-O,241,560, WO87/03,887 and EP-A-O,237,294).
The reaction of aluminum trialkyls with water in inert hydrocarbons is here mentioned as a known preparation process for alkylaluminoxanes. Mainly, however, other methods are mentioned as preferred for the preparation oligomeric methylaluminoxanes (MAO) from trimethylaluminum (TMA), since it is known from the literature that, according, to the preparation procedure described in more detail in, for example, U.S. Pat. No. 3,242,099, MAO can be prepared only with difficulty and in a very poor yield by slow addition of water to trimethylaluminum (TMA) (EP-A-O,108, 339); in addition, products are then obtained which, together with the transition metal component, do not give highly active catalyst systems (EP-B-O,069,951).
In this connection, it is stated explicitly in J. Polymer Science, 23, No. 8 (page 2120): "Simple synthetic routes to the methylaluminoxane [O-Al(CH.sub.3)-].sub.n are not available owing to the extremely high reactivity of the parent trimethylalane. This notwithstanding, the synthesis through direct reaction between Al(CH.sub.3).sub.3 and H.sub.2 O in a 1:1 molar ratio in toluene solution has been reported. We found this method not very reliable. The degree of oligomerization of the resulting aluminoxane was scarcely reproducible and the reaction rather uncontrollable."
These shortcomings were to be eliminated by reacting trimethylaluminum (TMA) with salts containing water of crystallization, such as aluminum sulfate hydrate (EP-A-O,108,339), or generally hydrates of salts of a type which are not reduced under the reaction conditions (EP-A-O, 208,561), or, in another procedure, by reacting TMA with inorganic substances which contain water bound by absorption or adsorption, such as finely dispersed silica (WO-A-89/02, 453), alumina (WO-A-89/02,453), hydrated alumina (EP-A-O,315,234) or molecular sieves (doctorate thesis I. Herwig, Hamburg University, 1979).
The last-mentioned preparation methods involve, of course, additional expense on equipment and operating costs; they have the considerable disadvantage that the solids used must as a rule be comminuted and screened (especially salts containing water of crystallization), so that they can readily be metered, and in addition their water content must be precisely adjusted and controlled for a specific and reproducible reaction. Moreover, long reaction times and, at least at the start of the reaction, frequently low temperatures are required (EP-A-O,315,234, WO-A-89/02,453), which results not only in correspondingly lower space/time yields, but also in higher expense on equipment and/or energy.
There was therefore a demand for a simple process for preparing oligomeric MAO, which gives a good yield of product which is soluble in inert hydrocarbons and with specific transition metal compounds, gives highly active catalyst systems for the polymerization of olefins.